Domestically, there are currently about 450 towered airports in the United States that do not have any significant surveillance capabilities to prevent potentially unsafe travel conditions. While there are surveillance technologies available to these airports, such as radar (for example X-band radar), millimeter wavelength radar, multilateration, ADS-B capabilities, and virtual out-the-window capabilities based on camera technologies, these technologies are expensive and cannot be economically justified by small to medium-sized airports. Further, some of these technologies require equipage by vehicles and aircraft; thus, unequipped targets would be unseen by even by some of these technologies. This leaves small to medium-sized airports without any way to reliably detect and track vehicles and aircraft on the airport surface.